1. Field of the Invention
The present invention relates to a driver circuit for modulation of an injection laser at a high repetition rate.
2. Description of the Prior Art
Semiconductor lasers, also known as injection lasers, are used for various systems, such as laser recorders and fiber optics communications to provide a light source which is representative of digital information. Laser modulator driver circuits are used to transform the digital information, in the form of pulse coded modulation (PCM), to a light source from the injection laser whose intensity is representative of the received PCM.
Laser modulators capable of high repetition rates which employ bipolar devices and charging circuits are well known. Such a modulator is described in U.S. Pat. No. 4,027,179, entitled "High Repetition Rate Injection Laser Modulator," issued to Hirohisa Kawamoto, et al., May 31, 1977.
Driver circuits for laser modulators capable of high repetition rates employ techniques to minimize delay from the application of an electrical signal to the laser and the laser corresponding light output. The laser is arranged in a prebiased condition with a current which is near the laser threshold current, that is, the current at which the laser will initiate its lasing action. Application of an electrical signal to a laser in this prebiased condition improves the speed of response of the laser. Such a circuit is described in U.S. Pat. No. 4,009,385, entitled "Laser Control Circuit," issued to Darrell Dean Sell, Feb. 22, 1977.
It is also well known that the speed of response of a data input device, such as a bipolar transistor, is improved by prebiasing the transistor. A d.c. voltage is applied to the gate element of the bipolar transistor. In this bias arrangement, a capacitor or isolation diodes connected between the gate element and the source means that supplies digital signals for modulating the injection laser is normally provided for d.c. isolation between the input gate element and the source means. However, with capacitive coupling the d.c. voltage level of the input signal shifts as the repetition rate and pulse width of the input signal changes. Still further, the speed of response of the modulator decreases due to the corresponding charging time of the capacitor or isolation diode.
A circuit for modulating a diode laser with an FET is described in "Electronics Letter," dated Oct. 16, 1975, pages 517 and 518, in an article entitled "Direct Modulation of D.H. GaALAs Lasers with GaAs M.E.S.F.E.T.S.". The circuit including an resistance-capacitance network utilizes a negative pulse of a return-to-zero pulse train to terminate conduction of a MESFET and cause lasing of a forward d.c. biased injection laser.